....don't be down on yourself as your not stupid, you just haven't learned how to weld yet.
....ok, that being said, first off start practicing welding in the flat position on one piece of metal. Just run practice beads, one after the other until they look good. Then go on to welding two pieces together. Butt joints, lap joints, etc. After you've got this down, then you go onto welding in the horizonal position, then verticle & then on to overhead welding.
When you prepare two pieces of metal with a 'V' you put what is called a 'land' at the bottom of the 'V' on both pieces. This gives you some 'parent metal' for back up. I'd have to see exactly what's going on to be more specific but if the frame is already cut & fitted for welding and you have a big gap you might try putting a small piece of metal behind {on the back side} of the 'V' for back up to prevent burn through. You can grind it off later if you want. Ideally with a 'V' prepared joint you want a little gap for good penitration. As you are welding the front side of the joint it is burning through to form a perfect bead on the back side {like the root pass on a pipe weld}. BUT this is getting WAY ahead of where you need to be when starting out learning how to weld. It'd be like learning how to fly in a 747 jet! Anyway, so start out by practicing the basics.
For your specific questions; The welding machine you have is a great stick welder. 6013 is good rod & 1/8" is a good size. Although you might try 3/32" rod as it's a little smaller than the 1/8" and hence is easier to control. Also you run it at a lower setting so it won't burn through as easy. The way you are practing with scrapes is a great idea, as long as you also have the basics down pat...
If the 'V' is deep you just make multipul passes, you don't go slower. So yes, make a pass, clean it up & then another pass. Also if you let it cool between passes it won't burn through as easy. Don't cool it with water though cuz it'll make it brittle.
You should probably get a welding book or go on line to look for diagrams to help you. Or there is a really good welding web site. I don't remember the name of it but a guy on this site has mentioned it before. His name is 'big truck driver' {I think}....
Practice & you'll get it down..... hope this helps ya, Bill